Optical Coherence Tomography (OCT) is a non-contact imaging system and method which may be used for surface and thickness profiling of optical structures and assemblies, for example, contact lenses or contact lens subassemblies. Using special Fourier-domain OCT (FDOCT) with an ultra-long scan range, the entire cross-section of an object, for example a contact lens submerged in saline, can be imaged in a single sweep (B-scan).
The current method of processing the raw image data from a B-scan requires the conversion of frequency to depth using fast Fourier transform (FFT). This usually results in a processed image containing two mirror images—a real image and a virtual image, depending on the pathlength of an internal reference beam. The pathlength of the reference beam may be adjusted continuously, and there are two particular positions such that one position results in a real image of the full cross-section while the other position results in a mirrored virtual image of the full cross-section. The virtual image is normally referred to as the complex conjugate artifact of FDOCT. In terms of signal value vs. pathlength, the virtual image appears as a mirror image of the real image, with the mirror point at zero pathlength.
A characteristic of the above image capture system is the systemic (monotonic) change in signal intensity with scan depth. For the B-scan cross-section images, the pathlength of the reference beam translates into scan depth (in terms of optical path length of the scanning beam). For example, for a given beam focus position and for a given medium absorption level, the intrinsic signal degradation with depth is the dominating factor. If an object such as a contact lens is placed in a cuvette and is scanned from the top of the lens, the apex of the lens will appear near the top (depth=0) of a B-scan in the real image, resulting in a higher signal intensity at the apex. Conversely, the edge of the lens will appear near the top of the B-scan in the virtual image, resulting in a higher signal intensity at the edges, rather than the apex. As a result, when the real and virtual images are viewed together, the overall image quality is reduced, and neither individual image contains the optimal signal level for the entire sample.
It is therefore desirable to provide for a method and system for capturing the real and virtual images of FDOCT in sequence or consecutively to provide for increased signal intensity at various portions of the object being imaged, for example at the apex and edge of an object such as a contact lens.